It is disclosed in U.S. Pat. No. 4,152,499 that polymer obtained by polymerizing pure isobutene using boron trifluoride complex catalyst, is butene polymer containing double bonds of vinylidene structure in a high ratio and that maleic modification using maleic anhydride or the like can proceed efficiently. The maleic-modified polybutene disclosed in this patent gazette is accepted as being preferable in view of its performance, economy and environmental protection as compared with the conventional maleic-modified polybutene that has been obtained through chlorination process.
Accordingly, in recent years, there have been made many a proposal for producing polybutene having a large content of vinylidene-type double bonds, using boron trifluoride complex catalyst. Such butene polymer has excellent reactivity to maleic anhydride, in addition, it has also excellent adaptability to epoxidation or the like.
It is more advantageous in view of economy to use butadiene raffinate obtained from large-scale production rather than the use of 100% pure isobutene as a starting material in order to obtain polybutene having larger content of vinylidene-type double bonds. However, the use of butadiene raffinate as a starting material is not acceptable in that the amount of remained fluorine in polymer (hereinafter referred to as "residual organic fluorine") that is prepared in the presence of boron trifluoride catalyst, is large. As this residual fluorine is organic fluorine, if the butene polymer of this type is used as an additive to fuel oil, fluorine compounds are produced during the combustion of the fuel oil and fluorine compounds are released into the air to cause possibly air pollution.
Specifically, in the polymerization using boron trifluoride-methanol complex catalyst, when pure isobutene is used as a starting material, residual fluorine content is as low as 5 ppm or less, meanwhile if butadiene raffinate is used as a starting material, residual fluorine content is generally as high as 60 ppm or more, which is usually 90 to 120 ppm.
In view of air pollution, because polymer containing such a high content of residual fluorine as mentioned-above is not preferable, polymer of low fluorine content is demanded.
In U.S. Pat. No. 5,674,955; there is disclosed a method for reducing the content of residual organic fluorine to a level lower than 40 ppm by subjecting raw material of butene mixture to polymerization after reducing 20% or more of 1-butene content through catalytic hydroisomerisation. According to the disclosure on examples, however, even when 1-butene content in feed material is reduced from 22% to 5% by weight, the organic fluorine content in polymers is barely reduced to 17 ppm. Furthermore, this method necessitates one additional process in order to reduce the content of 1-butene as pretreatment of starting material.
PCT Publication WO 96140808 discloses a method for reducing organic fluorine content by dividing polymerization process into plurality of steps. However, this proposal is also economically unfavorable in view of costs for both equipment and operation because this also necessitates additional processes likewise the above-mentioned method.
It is an object of the present invention to provide an efficient process for preparing butene polymer having high reactivity in maleic-modification and epoxidation and the content of residual fluorine being reduced to an acceptable level for practical use without accompanying substantial increases in investment in equipment and also in additional cost for starting material.